Performance Studies of CuxFe1-XF2 Conversion Cathode Materials

Abstract

Lithium metal/metal fluoride batteries are potentially more energy dense compared to traditional lithium intercalation batteries; the redox chemistry in conversion materials involves multi electron transfer processes per transition metal atom (e.g., Cu2+ to Cu0, Fe2+/Fe3+ to Fe0) while intercalation of traditional electrodes involves a single electron transfer per transition metal atom. CuF2 is of interest as an active cathode material due to its high theoretical potential of 3.55 V [1] [2] [3]. Along with its theoretical specific capacity of 528 mAh/g, CuF2 provides a high theoretical energy density of 1874 Wh/kg. However, due to Cu dissolution, CuF2 is not fully reversible and has poor cycle life [2]. On the other hand, FeF2 has good cyclability but suffers from large hysteresis due to different reaction pathways during charge and discharge [4]. By substituting Cu into the Fe lattice, CuxFe1-xF2, a ternary metal fluoride, shows promise overcoming the issues with CuF2 and FeF2. Upon charge of CuxFe1-xF2 electrodes, initial reconversion of Fe to Fe2+ and Fe3+ is followed by reconversion of some of the Cu to Cu2+ while the remaining Cu provides conductive pathways during the charge process [5]. Studies have been performed on the Cu0.5Fe0.5F2 which showed improvement over the binary CuF2 and FeF2. The Cu-substituted material showed low hysteresis (<150mV) which allows higher cycling efficiency [5]. In this work, we present further electrochemical testing on Cu0.5Fe0.5F2 as well as investigations of Fe and Cu rich CuxFe1-xF2. The ternary metal fluorides are manufactured by mixing FeF2 and CuF2 in a high-energy ball mill until a solid solution is formed. The structure and phase purity of the materials are characterized by XRD while XPS is used to evaluate the chemistry of the solid electrolyte interfaces. Reference [1] A. N. Mansour, F. Badway, W. S. Yoon, K. Y. Chung and G. G. Amatucci, "In situ X-ray absorption spectroscopic investigation of the electrochemical conversion reactions of CuF2-MoO3 nanocomposite," J. Solid State Chem., vol. 183, pp. 3029-3038, 2010. [2] X. Hua, R. Robert, L.-S. Du, K. M. Wiaderek, M. Leskes, K. W. Chapman, P. J. Chupas and C. P. Grey, "Comprehensive Study of the CuF2 Conversion Reaction Mechanism in a Lithium Ion Battery," J. Phys. Chem. C, vol. 118, pp. 15169-15184, 2014. [3] F. Badway, A. N. Mansour, N. Pereira, J. F. Al-Sharab, F. Cosandey, I. Plitz and G. G. Amatucci, "Structure and Electrochemistry of Copper Fluoride Nanocomposites Utilizing Mixed Conducting Matrices," Chem. Mater., vol. 19, pp. 4129-4141, 2007. [4] J. K. Ko, K. M. Wiaderek, N. Pereira, T. L. Kinnibrugh, J. R. Kim, P. J. Chupas, K. W. Chapman and G. G. Amatucci, "Transport, Phase Reactions, and Hysteresis of Iron Fluoride and Oxyfluoride Conversion Electrode Materials for Lithium Batteries," ACS Appl. Mater. Interfaces, vol. 6, pp. 10858-10869, 2014. [5] F. Wang, S.-W. Kim, D.-H. Seo, K. Kang, L. Wang, D. Su, J. J. Vajo, J. Wang and J. Graetz, "Ternary metal fluorides as high-energy cathodes with low cycling hysteresis," Nat. Commun., pp. 1-9, 2015.